As a step in the process of the electrolytic refining of zinc from zinc ore concentrate, iron, which is usually present in substantial quantity, is separated from the zinc by precipitation of complex basic iron sulphate from a solution of iron and zinc sulphate (plus minor components) in the presence of ammonium, potassium or sodium ions. These iron sulphate compounds are very similar to the jarosite found in nature and may be and hereinafter are referred to as jarosite residues, this term including, also, other metal sulphates and materials filtered out with the jarosite compounds. What may be called the "Jarosite Process" in zinc refining is described in a book publication entitled, AIME World Symposium on Mining & Metallurgy of Lead & Zinc, Volume II, Extractive Metallurgy of Lead and Zinc, published in 1970 by The American Institute of Mining, Metallurgical and Petroleum Engineers, Inc., Library of Congress Catalogue Card Number 78-132404, beginning on Page 229. The precipitation iron-zinc separation process resulting in the formation of the jarosite; i.e., the jarosite process, is also described in U.S. Pat. No. 3,434,947 which is incorporated herein by reference.
The jarosite residues have heretofore been regarded and disposed of as an entirely useless waste product. Produced in very copious quantities, the disposal of this material has presented problems involving the acquisition and use of dumping ground and the very substantial cost of handling the material.
The ammonium jarosite produced by the separation precipitation in the presence of ammonium ions may be represented by the formula: (NH.sub.4).sub.2 Fe.sub.6 (SO.sub.4).sub.4 (OH).sub.12. In addition to ammonia and iron in the jarosite, the residue also contains zinc and manganese and generally copper and other trace metals. These elements are useful plant nutrients and the jarosite residues offer substantial values as fertilizer. The discovery of the value of these residues converts a troublesome, heretofore wasted by-product to a useful and valuable material.
An object of the invention is to provide a modified jarosite process of electrolytic refining of zinc wherein the production of a useful jarosite residue fertilizer is integrated with the known refining process. With the addition of superphosphate, a fertilizer containing nitrogen, phosphorus and potassium, as well as useful metallic trace elements, is produced. Sulphuric acid, formed as a by-product in the roasting of the zinc ore concentrates, may be used in the treatment of phosphate rock or bone to produce the superphosphate in a further integration of the fertilizer production and refining process.
Another object, important in certain areas of the world, is to recover process water which may be reused. In another aspect of the invention, iron is recovered from the jarosite residue by fractional crystallization or iron sulphate from the redissolved sulphate solution, leaving the balance of the iron in the residue for fertilizer utilization. The separated iron sulphate may be roasted to drive off sulphur oxides and the iron oxides so produced may be further treated to produce metallic iron. The sulphuric oxides are returned to the acid plant of the refinery.
Other objects and the advantages of the invention will become apparent as the description thereof proceeds.